1. Field of the Invention
The present invention relates to a paper feeding apparatus for feeding stacked paper sheets (plain paper, coat paper, an OHP (Over Head Projector) sheet, glossy paper, a cut sheet such as a glossy film) one sheet at a time beginning with the uppermost paper sheet, and more particularly to a paper feeding apparatus suitable for use in a printer.
The present invention relates to a printer having the above-mentioned paper feeding apparatus, which, one by one, feeds stacked paper sheets beginning with the uppermost paper sheet, so as to print the fed paper sheets. More particularly, the present invention relates to a technique for preventing diagonal conveyance of a fed paper sheet with respect to the conveying direction.
2. Related Art
An example of the schematic structure of a usual printer is shown in FIG. 13.
Referring to FIG. 13, reference numeral 1 represents a paper feeding apparatus for, one by one, feeding paper sheets P. The fed paper sheet P is conveyed such that it is held by a pair of conveying rollers 2 and 3, and then printed by a printing means 4 and discharged to the outside of the apparatus by a pair of discharge rollers 5 and 6.
As the paper feeding apparatus 1, a structure as shown in FIG. 14 is available (refer to Japanese Utility Model Publication No. Hei. 8-3396). Referring to the figure above, reference numeral 10 represents a paper feeding roller having a D-shape side cross section and composed of a circular-arc portion 10a and a straight portion 10b.
Reference numeral 11 represents a guide block. A shaft 10c of a paper feeding roller 10 is supported by the guide block 11.
Reference numeral 12 represents a cassette including a stacking plate 12a. A plurality of paper sheets P are, in a stacked state, set on the stacking plate 12a. Reference numeral 12c represents a spring. The spring 12c urges the paper sheet P toward the paper feeding roller 10.
Reference numeral 13 represents a separation pad attached to the upper surface of a bracket 13a. The separation pad 13 is disposed in a rotation locus for the circular-arc portion 10a of the paper feeding roller 10 and, by a spring 14, urged in a direction of the shaft 10c along the guide 15.
Reference numeral 16 represents an idle roller rotatively attached to the guide block 11. Reference numeral 17 represents a movable idle roller having a shaft 17a movably received within an elongated groove 11a of the guide block 11. The movable idle roller 17 is, by a spring 18, urged toward the separation pad 13 so as to be in contact with the separation pad 13.
Urging force F2 of the spring 18 is made to be smaller than urging force F1 of the spring 14 of the separation pad 13 (that is, F1&gt;F2).
The paper feeding apparatus structured as described above is operated as follows.
In a standby mode, a straight portion 10b of the paper feeding roller 10 is placed opposite to the paper sheet P, as shown in FIG. 14, such that the paper feeding roller 10 is not in contact with the paper sheet P. Since the urging force F2 of the spring 18 of the movable idle roller 17 is smaller than the urging force F1 of the spring 14 of the separation pad 13, the movable idle roller 17 is pushed upwards by the separation pad 13. Thus, its shaft 17a is in contact with the top end of the elongated groove 11a.
When the paper feeding operation is performed, the paper feeding roller 10 is rotated in a direction indicated by an arrow. When the circular-arc portion 10a is brought into contact with the uppermost paper sheet P1 among the paper sheets P, the paper sheet P1 is fed toward the separation pad 13. Since a paper sheet P2 next to the paper sheet P1 are attracted to one another due to the static electricity, and since frictional force acts between the paper sheet P1 and the paper sheet P2, the paper sheet P2 is sometimes fed together with the paper sheet P1.
However, the paper sheet P2 is separated from the paper sheet P1 by the separation pad 13 as follows so that only the uppermost paper sheet P1 is fed.
That is, the movement of the paper sheet P2 is prevented because its leading end is brought into contact with the separation pad 13 so that the paper sheet P2 is primarily separated from the paper sheet P1.
Assuming that the frictional force between the circular-arc portion 10a of the paper feeding roller 10 and the paper sheet P1 is f1, the frictional force between the paper sheet P2 and the separation pad 13 is f2 and the frictional force between the paper sheet P1 and the paper sheet P2 is f3, the paper feeding roller 10 and the separation pad 13 are structured to establish the relationships f1&gt;f2&gt;f3. Therefore, when both of the paper sheet P1 and the paper sheet P2 have been brought to a state where they are held by the circular-arc portion 10a of the paper feeding roller and the separation pad 13 attributable to the rotation of the paper feeding roller 10, the movement of the paper sheet P2 is prevented by the frictional force generated between the paper sheet P2 and the separation pad 13. Thus, the paper sheet P2 is secondarily separated from the paper sheet P1 so that only the paper sheet P1 is fed. Since the separation pad 13 is disposed in the rotation locus of the circular-arc portion 10a of the paper feeding roller 10, the rotation of the circular-arc portion 10a causes the separation pad 13 to be pushed downwards by the circular-arc portion 10a. However, the movable idle roller 17, which is urged toward the separation pad 13 by the spring 18, is brought into contact with the separation pad 13 even if the separation pad 13 has been pushed downwards. Also the thus-maintained contact causes the paper separation operation to be performed.
After the paper feeding roller 10 has been rotated one time, the standby state (the state shown in FIG. 14) is restored.
As described above, only the uppermost paper sheet P1 is fed.
When the above-mentioned paper feeding apparatus (shown in FIG. 14) is employed by a printer structured, for example, as shown in FIG. 13, the fed paper sheet P1 is conveyed while being held between the pair of the conveying rollers 2 and 3, and then printed by the printing means 4. When conveyance of the paper sheet P1 has been started by the pair of the conveying rollers 2 and 3, the paper sheet P1 has not completely been separated from the paper feeding apparatus in general. That is, the paper sheet P1 is in a state where its trailing end is held by the separation pad 13 and the movable idle roller 17.
Therefore, the paper sheet P1 is conveyed by the pair of the conveying rollers 2 and 3 in a state (a state where the paper sheet P1 is pulled rearwards, that is, a state where the paper sheet P1 is applied with a back tension) where the paper sheet P1 bears the load applied from the holding portion until the trailing end of the paper sheet P1 passes through the holding portion between the separation pad 13 and the movable idle roller 17. The reason why the structure is employed in which the movable idle roller 17 is urged toward the separation pad 13 to hold the paper sheet P1 between the movable idle roller 17 and the separation pad 13 lies in that conveyance of the paper sheet P2 together with the paper sheet P1 when the paper sheet P1 is attempted to be fed must be prevented, the conveyance being performed due to the adsorption of the paper sheet P2 to the paper sheet P1 attributable to the static electricity or frictional force generated between the paper sheet P1 and the paper sheet P2.
However, the above-mentioned paper feeding apparatus encounters enlargement of the back tension because the load in the holding portion between the separation pad 13 and the movable idle roller 17 is too large.
When the paper feeding roller 10 has been rotated one time for feeding the paper and thus the state (the state where the circular-arc portion 10a does not press the separation pad 13) shown in FIG. 14 has been realized as described above, the above-mentioned paper feeding apparatus, set such that the urging force F1 of the spring 14 of the separation pad 13 is larger than the urging force F2 of the spring 18 of the movable idle roller 17, causes the separation pad 13 to be brought to a state of stoppage such that the movable idle roller 17 has been pushed upwards until the shaft 17a of the movable idle roller 17 is brought into contact with the top end of the elongated groove 11a.
That is, the foregoing paper feeding apparatus is structured such that the paper sheet P1 is conveyed by the pair of the conveying rollers 2 and 3 in a state where the trailing end of the paper sheet P1 is held between the separation pad 13 and the movable idle roller 17 by the urging force F1 of the spring 14 of the separation pad 13.
Since the separation pad 13 is structured to hold the paper sheet between the separation pad 13 and the circular-arc portion 10a of the paper feeding roller 10 so as to prevent conveyance of two or more paper sheets, the urging force F1 of the separation pad 13 must be relatively large (at least larger than the urging force F2 of the movable idle roller 17 as described above).
Therefore, since the above-mentioned paper feeding apparatus is structured such that the trailing end of the paper sheet P1 is held by the relatively large urging force F1, a great load is generated in the holding portion.
In a case where conveyance force capable of overcoming the above-mentioned load cannot be obtained by the pair of the conveying rollers 2 and 3, the paper feeding accuracy deteriorates. As a result, the printing accuracy realized by the printing means 4 deteriorates.
Therefore, the foregoing paper feeding apparatus must enlarge the paper holding force which is realized by the pair of the conveying rollers 2 and 3 in order to obtain the conveyance force sufficiently overcoming the above-mentioned load. Moreover, a great drive power is required to operate the pair of the conveying rollers 2 and 3. As a result, the size of the apparatus is enlarged and the electric power consumption is also enlarged. What is worse, the pair of the conveying rollers 2 and 3 are easily worn.
A paper sheet which is fed by the paper feeding apparatus is sometimes fed diagonally relative to the conveying direction. Since the fed paper sheet is usually conveyed by the pair of the conveying rollers 2 and 3 and printed by the printing means 4 as shown in FIG. 13, the diagonal conveyance of the paper sheet results in printing being performed diagonally. Therefore, the diagonal conveyance of the paper sheet must be prevented.
As a technique for preventing the diagonal conveyance of a paper sheet, a technique as shown in FIGS. 15 (a) and 15 (b) is known in which a paper sheet is fed by a single paper feeding roller 7; and then the paper feeding roller 7 is stopped after the leading end P1a of the fed paper sheet P1 has allowed to pass through a nipping portion (the holding portion) N between the pair of the conveying rollers 2 and 3, followed by temporarily and inversely rotating the pair of the conveying rollers 2 and 3.
If the pair of the conveying rollers 2 and 3 are rotated inversely in a state where the paper sheet P1 is conveyed diagonally, the timing, at which the leading end P1a of the paper sheet passes through, in the opposite direction, the nipping portion N between the pair of the conveying rollers 2 and 3, is different for each widthwise edge of the paper sheet. In a case where the paper sheet P1 is conveyed diagonally, for example, as indicated by an imaginary line shown in FIG. 15 (a), the leading end P1a1 which is the upper portion when viewed in FIG. 15 (a) passes through the nipping portion N, and then a leading end P1a2, which is the lower portion, passes through the same. That is, a state is realized in which the conveying force generated by the pair of the conveying rollers 2 and 3 does not act on the leading end P1a1 which is the upper portion and the conveying force acts on the leading end P1a2 in the lower portion.
Since the paper feeding roller 7 is stopped in the above-mentioned state, the paper sheet P1 is rotated in a direction indicated by an arrow X shown in FIG. 15 (a). As a result, the leading end P1a of the paper sheet is arranged along the nipping portion between the pair of the conveying rollers 2 and 3.
Then, the pair of the conveying rollers 2 and 3 are rotated forwards so that the paper sheet P1 is conveyed straight.
Some paper feeding apparatuses for the printers have a structure such that a paper feeding tray 12' is, as shown in FIG. 16 (b), positioned diagonally to reduce the overall area required for installing the printer.
If the paper feeding tray 12' is diagonally disposed as described above, the deadweight of the paper sheet P1 acts in the conveying direction during the conveyance. Therefore, if a single paper feeding roller is employed, the paper sheet can easily be conveyed diagonally.
Accordingly, an apparatus of the foregoing type has been structured such that at least a pair of the paper feeding rollers 7' are provided as shown in FIG. 16 (a) to prevent diagonal conveyance when the paper sheet is conveyed. However, if one pair of the paper feeding rollers 7' are provided, the above-mentioned technique for preventing diagonal conveyance (see FIGS. 15 (a) and 15 (b)) cannot prevent the diagonal conveyance of the paper sheet. Since the pair of the paper feeding rollers 7' are attached to a common drive shaft 7a to simultaneously press the paper sheet P1, the inverse rotation of the pair of the conveying rollers 2 and 3 cannot rotate the paper sheet P1.
If the pair of the paper feeding rollers 7' are attached to individual drive shafts to enable the paper feeding rollers 7' to be rotated independently and freely when the pair of the conveying rollers 2 and 3 are rotated inversely, the above-mentioned technique (see FIGS. 15 (a) and 15 (b)) for preventing the diagonal conveyance is able to prevent the diagonal conveyance of the paper sheet. However, if the pair of the paper feeding rollers 7' are attached to the individual drive shafts, the two drive shafts must be synchronized with each other to prevent excessive diagonal conveyance of the paper sheet when the paper sheet is fed. In order to make the paper feeding rollers 7' capable of independently and freely rotating when the pair of the conveying rollers 2 and 3 are rotated inversely, a clutch and so forth are required. Thus, the structure becomes too complicated.
Therefore, the structure, in which the pair of the paper feeding rollers 7' are attached to the individual drive shafts to make the paper feeding rollers 7' capable of independently and freely rotating when the pair of the conveying rollers 2 and 3 are rotated inversely, is not an advantageous structure.
On the other hand, the structure formed as shown in FIG. 14, which is provided with the paper feeding roller 10 having a D-shape side cross section and the movable idle roller 17 capable of independently and freely rotating with respect to the paper feeding roller 10, is considered to be able to use the above-mentioned diagonal conveyance preventive technique (see FIGS. 15 (a) and 15 (b)). If the pair of the conveying rollers are rotated inversely in a state where the paper feeding roller 10 is not pressing the paper sheet, the rotation of the movable idle roller 17 also serving as a load is considered to be capable of rotating the paper sheet.
However, the structure shown in FIG. 14 cannot easily and smoothly rotate the paper sheet, that is, smoothly prevent the diagonal conveyance and cannot rotate the same because of the following reason.
Since the structure shown in FIG. 14 is formed such that the urging force F1 of the spring 14 of the separation pad 13 is larger than the urging force F2 of the spring 18 of the movable idle roller 17, the separation pad 13 is brought into a state of stoppage in which the separation pad 13 has upwardly pushed the movable idle roller 17 until the shaft 17a of the movable idle roller 17 is brought into contact with the top end of the elongated groove 11a when the paper feeding roller 10 has been rotated one time to be brought to the state shown in FIG. 14 (the state in which the circular-arc portion 10a does not press the separation pad 13).
If the pair of the conveying rollers are rotated inversely to rotate the paper sheet P1 in the foregoing state, the paper sheet P1 is attempted to be rotated in a state where its trailing end is held between the separation pad 13 and the movable idle roller 17 by the urging force F1 of the spring 14 of the separation pad 13.
However, since the separation pad 13 is arranged to hold the paper sheet between the separation pad 13 and the circular-arc portion 10a of the paper feeding roller 10 so as to prevent conveyance of two or more paper sheets as described above, the urging force F1 of the separation pad 13 is relatively large (at least larger than the urging force F2 of the movable idle roller 17 as described above).
Therefore, the paper sheet P1 must be rotated in a state where the trailing end of the paper sheet P1 is held as described above by the relatively large urging force F1. As a result, the paper sheet P1 cannot easily be rotated.
That is, the structure shown in FIG. 14 cannot easily prevent the diagonal conveyance of a paper sheet.